Vibrational cooling of monomeric bacteriochlorophylls in reaction centers of purple bacteria studied by time-resolved fluorescence spectroscopy

Photosynthesis in bacteria, algae, and plants begins with the absorption of light energy by (bacterio)chlorophyll molecules, part of which is then converted into heat, leading to a transient change in molecular temperature. We investigated this phenomenon in reaction centers (RCs) of the purple bacterium Rhodobacter (Rba.) sphaeroides using picosecond fluorescence spectroscopy. Exclusion of charge separation processes using the VR(L157) mutation allowed us to record the spectral dynamics of fluorescence of monomeric BChl a molecules. We found that excitation of RCs into the Soret band results in significant heating of BChl a by ~160 K with subsequent vibrational cooling, which manifests itself in a dynamic narrowing of the BChl a fluorescence spectrum with two characteristic times of 5 and 16 ps. The weaker heating by ~65 K and cooling with a characteristic time of 7.5 ps are observed upon excitation of RCs into the Q_x band. Excitation into the Q_у band does not result in any noticeable heating of BChl a. Difference absorption spectroscopy of tryptophan in the 280 nm region showed that the observed dynamics of the BChl a fluorescence spectrum are not associated with the dielectric rearrangement of the RCs protein matrix. Analysis of the obtained data using the phenomenological model of vibrational cooling led to the conclusion that during heat diffusion from excited BChl a, several amino acid residues from the immediate environment of BChl a act as the first solvation shell (FSS). At the first, faster stage, heat is transferred from BChl a to FSS, and at the second stage, FSS transfers heat to the protein matrix of RCs. Our work has shown the importance of taking into account vibrational cooling when studying the primary processes of photosynthesis.